This application relates generally to turbine blade airfoils and, more specifically, to methods of drilling cooling holes in turbine blade airfoils.
At least some known turbine blade airfoils use “film cooling” to cool turbine blades during operation of a gas turbine. Film cooling involves channeling cool air from an interior cavity of the turbine blade through air holes formed in the surface of the blade. Thin streams of cool air flow along an external surface of the airfoil to create a cool air buffer between the surface and the high-temperature gases produced during operation.
Known cooling holes are generally formed as a thin aperture or “channel” that is oriented obliquely relative to the surface of the airfoil, and that extends from an interior cavity to the airfoil surface. The desired properties of cooling holes require precision in machining that prohibits some known methods of machining. One known method of forming cooling holes uses electrical discharge machining, or “EDM” drilling. During EDM drilling, an airfoil surface, the “workpiece electrode” and a tool, i.e., the “tool electrode”, are submersed in a dielectric fluid. Current is induced between the tool and the work piece to remove material from the workpiece. The tool is then extended through airfoil surface to create the channel. The EDM process enables material to be removed from the airfoil in a precise and computer-controllable manner.
In some applications, it is desirable to form a low-angle channel, i.e., a channel in which the angle of the channel relative to the surface is small, such as less than 30°. However, forming channels at such a shallow angle may be difficult using conventional drilling methods, which may cause alignment difficulties such as slipping or skipping of the working end of the tool across the surface of the workpiece when starting the machining operation.